PySide2 memory leak during canvas paintEvent

[simon-kraeusel]

Bug report

A memory leak occurs during every frame draw when PySide2 is used with Python3. This issue was already fixed for PySide in the method matplotlib.backends.backend_qt5agg.FigureCanvasQtAgg.paintEvent
However, an additional check for QT_API == 'PySide2' is needed.

Code for reproduction

import matplotlib
matplotlib.use('Qt5Agg')
import matplotlib.pyplot as plt
plt.plot([1,2],[1,2])
plt.show()

Actual outcome

When a redraw of the figurecanvas is forced, the program memory increases steadily. For example during resizing the popup window.

Expected outcome

Constant memory during ui execution.

Matplotlib version

• Operating system: Windows 10
• Matplotlib version: 2.2.2
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg
• Python version: 3.7

all packages are pip installed

[simon-kraeusel]

The fix to this issue is:

if QT_API in ('PySide', 'PySide2') and six.PY3:
    ctypes.c_long.from_address(id(buf)).value = 1

to be inserted into
matplotlib.backends.backend_qt5agg.FigureCanvasQtAgg.paintEvent

I am new to github so don't know how to create a branch/commit/pull request sensibly. I will look into it and try to commit this fix.

[anntzer]

Indeed looks like PySide2 also suffers from the same issue (https://bugreports.qt.io/browse/PYSIDE-140?focusedCommentId=403528&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-403528), so I agree with the fix.

[dabana]

@simon-kraeusel checkout the matplotlib developper guide for a step by step guide how to contribute.

[simon-kraeusel]

@dabana Thank you. I sort of managed to get the fix into a merge request. However, I will have a closer look at the developer-guide in order to do it properly next time. Cheers.